Add GetExecutableDirectory to libbase

Tests will often want to get the executable directory in order to
find test data.

Test: out/host/linux-x86/nativetest64/libbase_tests/libbase_tests
Change-Id: Ica9d211bcd039fcf83a22fd494816abd01b97aa3
diff --git a/base/file_test.cpp b/base/file_test.cpp
index bbd5037..1021326 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -159,6 +159,14 @@
 #endif
 }
 
+TEST(file, GetExecutableDirectory) {
+  std::string path = android::base::GetExecutableDirectory();
+  ASSERT_NE("", path);
+  ASSERT_NE(android::base::GetExecutablePath(), path);
+  ASSERT_EQ('/', path[0]);
+  ASSERT_NE('/', path[path.size() - 1]);
+}
+
 TEST(file, GetExecutablePath) {
   ASSERT_NE("", android::base::GetExecutablePath());
 }